Hi, popped round a mates whos having probs with hot water upstairs. Downstairs hot water is fine. But in upstairs bathroom, bath hot tap runs hot for a min, then cold. Basin shower taps do not get hot, neither does thermostatic shower. Boiler is a vaillant turbo max combi. I cannot figure out whats wrong. No error codes. But pressure on CH gauge showing as 2.5 bar, even though its not running.